1. Field of the Invention
The present invention relates to automated apparatus for expediting the handling, processing and quality inspection procedures involved in the manufacture of nuclear fuel rods.
2. Description of the Prior Art
A nuclear fuel rod is comprised of a column of fuel pellets sealed in an elongated cladding tube typically formed of a zirconium alloy. In the process of manufacturing nuclear fuel rods, the last major operation prior to loading the tubes with fuel pellets is to seal off one of their open ends by welding an end plug thereto in a first weld operation.
Commonly assigned Schoenig, Jr. et al., U.S. Pat. No. 4,857,260, issued Aug. 15, 1989, is directed to automated apparatus for assembling and welding first end plugs to cladding tubes and then quality inspecting each weld. As disclosed therein, empty cladding tubes are placed on an input queue from which they are picked up one-by-one by a tube transporter for a stepwise indexing movement in a direction transverse to their axes into axial alignment with a succession of operating stations. At each operating station, the transporter pauses while the tubes are concurrently, axially translated into the operating stations. The first in the succession of operating stations is a welding station where a first end plug is welded to a tube open end. The next station is a cooldown station where each first end plug weld is rapidly cooled. Next, a unique serial number imprinted on the first end plug is read in a reader station. Then, the first end plug weld is inspected for integrity in an ultrasonic inspection station. In a second inspection station, the weld diameter and end plug concentricity with the cladding tube are gauged for acceptability. Then, in a last inspection station, the characteristics of any interior zirconium liner are determined.
The data obtained from these various inspection stations are gathered by data acquisition means, collated on the basis of each end plug serial number, and compared against predetermined quality control standards to determine whether the first end plug welds are acceptable or rejectable. As a result of this determination, the tubes, upon arriving at a final, sorting station, are automatically segregated into accepted and rejected lots. The accepted tubes are then conveyed to automated apparatus operating to load fuel pellets into the cladding tubes through their open ends opposite from the first end plug. It then remains to weld a final end plug to the loading end of each cladding tube in a final weld operation.
Since the final end plug weld involves essentially the same welding and inspection operations as does the first end plug weld, it would be advantageous to automate the final end plug weld in the same manner. However there are several operations necessary in the final end plug weld that are not performed in the first end plug weld. One of these distinctive operations is the inclusion of a plenum spring in a plenum section of each cladding tube. The plenum spring is a high spring constant compression spring that acts between the final end plug and the column of fuel pellets in the cladding tube to maintain the pellets in interfacial contact. For many fuel rod designs, the plenum spring is combined in a retainer with a tube holding a getter material. It is critical that every fuel rod contain a plenum spring.
Another distinctive operation is to backfill the cladding tubes with from three to twenty atmospheres of an inert gas, such as helium, prior to sealing off the tube interior with the final end plug weld. This means that at least a sealing weld to finally close off the tube interior must be performed under high pressure. To avoid having to perform a high pressure girth or seam weld affixing the final end plug to the cladding tube open end, the final end plugs are provided with a small pressurization hole. Thus, the girth weld can be performed in an inert atmosphere at normal pressure, i.e., one atmosphere. Then in a subsequent operation, the final end plug end of the cladding tube is inserted into a separate weld box which is then sealed and flooded with helium to the requisite tube backfill pressure. Helium thus enters the tube interior through the final end plug pressurization hole. When pressure equalization between the tube interior and the welding chamber is achieved, the pressurization hole is welded closed. A final end plug weld should then be checked to ensure that helium is not leaking through the girth weld or the pressurization hole seal weld.
When a final end plug welding and inspection apparatus is automated in the manner disclosed in the above noted U.S. Pat. No. 4,857,260, the times that the cladding tubes spend at each operating station are of an equal duration since the cladding tubes are indexed by a common transporter. The length of this equal duration is determined by the station that takes the longest time to complete its operation. Thus, throughput of the apparatus can be improved if the duration at each station is reduced by balancing out the functions performed at the various stations. Manufacturing economies can then be achieved, even at the expense of increasing the number of operating stations.